Power amplifiers including switching power amplifiers are known. One figure of merit for power amplifiers is efficiency. Switching power amplifiers have been demonstrated that have excellent efficiency at audio frequencies.
However, most communication standards now specify some form of complex modulation (e.g. combination of phase and amplitude modulation) and often multiple signals or carriers. The net result of these carriers with complex modulation is a signal that may have a large dynamic range, such as 6–15 dB or more for a peak to average ratio. These communication standards often define systems intended to operate in the low GHz frequency bands with a large bandwidth, for example, 2.1 GHz with a 20 MHz bandwidth for wide band code division multiple access systems. Base station transmitters can have peak power requirements on the order of 250 watts.
Practitioners have resorted to class A or AB power amplifiers to implement radio frequency amplifiers that will satisfy the linearity and dynamic range requirements noted above. These A or AB amplifiers may have efficiencies on the order of 10%, e.g. 10 watts are consumed in order to provide 1 watt of output power. More recently switching power amplifiers that operate at high frequencies, such as radio frequencies have been considered in order to improve the efficiency for such amplifiers.
Various proposals have been advanced that utilize a delta sigma modulator (DSM) to provide the switching signal to drive the switching amplifier stages, however these approaches have their respective problems. For example, the DSM must be clocked at twice the radio frequency to meet the Nyquist criteria and in practice this is usually 4 or 8 times the radio frequency. It may be difficult or impractical to implement appropriate DSMs (devices and resonators) at these clock rates.
One approach that has been used for linear power amplifier stages where gain and phase characteristics are controlled is up-converting an intermediate frequency signal to a radio frequency, amplifying the radio frequency signal and down converting the amplified signal to form a feedback signal. This places the power amplifier inside a control loop where imperfections with the amplifier can be controlled or compensated. However due to loop stability issues this approach does not work for switching amplifiers where gains and phases are not well known.
Therefore, a need exists for an improved efficiency power amplifier, suitable for use at radio frequencies.